The invention is directed to an improved efficient, relatively simple, inexpensive, light-weight power supply having particular utility for energizing a low-voltage incandescent lamp from a conventional 115-volt AC line source.
In the design of illumination systems for such applications as theatrical lighting, photographic lighting, projection devices, and the like, selecting the appropriate lamp is critically important. For conventional incandescent lamps the selection is generally based on the following performance criteria:
a. Lamp power, which is proportional to the total illumination, that is, to the amount of light delivered by the lamp;
b. The characteristic color temperature of the lamp, namely, the color balance of the light emitted by the lamp, that is, the relative proportions of the light in each region of the visible spectrum. Color temperature is a critical parameter for photographic and projection applications, because it determines the accuracy of color rendition.
c. The configuration of the lamp filaments. Such filaments may be single or multiple, they may be disposed along the axis of the lamp, or transversely; they may be a straight wire, a helically coiled wire, or compound helically wound coils. For optimum illumination, the physical configuration of the filament must be matched to the geometry of the reflector used to direct the light. This often means that a particular reflector design must be specific to a given lamp type, and such a design is ineffective if a similar, but not exactly identical, lamp is used.
d. After parameters (a), (b) and (c) are selected, the operating voltage of the lamp must be considered. Under ideal circumstances, an incandescent lamp design could be specified independently by its power, color temperature, filament configuration and operating voltage. However, in actuality, the critical optical parameters of the lamp determine the operating voltage, so that when a fixed operating voltage is specified, the possible combinations of the other three optically relevant variables are constrained. The net result is that the best lamp for a particular application often operates at an inconvenient voltage.
With the exception of automotive products and battery-operated devices, most consumer electronic equipment are powered by the conventional line voltage (115 VAC, 60 Hz, single phase). Unfortunately, technically advanced illumination systems are seldom based on lamps which operate at this easily-obtainable Voltage. Far more common are applications for which the best lamp choice is a low-voltage design, requiring, for example, 12, 24 or 36 volts DC, or an equivalent AC voltage whose root-mean-square [RMS] voltage is equivalent to the nominal DC rating of the lamp.
Because of the conflict of the 115 VAC line voltage output, and a low-voltage DC lamp, the system designer has three conventional solutions at his disposal.
The first-and simplest solution is a power supply which uses a power transformer to step the line voltage down to the appropriate level. For example, a transformer wound for a 115 VAC primary and a 36 VAC (RMS) secondary will operate a 36-volt lamp directly with no other components. A potential disadvantage of such a power supply is the fact that the output voltage is unregulated, and any fluctuation of the line input voltage causes the voltage applied to the lamp to vary proportionally. Another disadvantage in the use of a transformer is that of weight. Transformers are made from iron and copper, and the amount of iron in the transformer is proportional to the amount of power the transformer must handle. For example, in order to drive a 400 watt lamp, the amount of iron required dictates a transformer weighing in the neighborhood of fourteen pounds. For many applications, such a weight cannot be tolerated.
The second conventional power supply available to the designer to reconcile a low-voltage lamp to 115 VAC line voltage is a linear DC power supply. The power supply consists a step down transformer, a full-wave rectifier bridge connected the secondary of the transformer which converts the stepped down to unfiltered DC, and a control stage which serves to regulate the DC voltage to the required value. Such a power supply overcomes the voltage variation shortcomings of the simple transformer approach, although at a cost of reduced efficiency due to increased heat losses in the circuit, and increased complexity. Moreover, the linear DC power supply still uses a transformer, so that the overall weight of the power supply includes the weight of the transformer and the additional weight of the regulation stage components.
A third conventional solution available to the designer of a power supply for driving a low-voltage lamp from the AC line is a switched power supply. Like the linear DC power supply, the output of the switched-mode power supply is a DC voltage. Unlike the linear DC power supply, the switched-mode power supply does not incorporate a transformer. Instead, unregulated full-wave rectified high-voltage DC is broken into short chopped packets of charge by means of a high speed series switch. The resulting chopped DC is smoothed into a nearly continuous flow of current by a large inductor/capacitor filter network in the output of the power supply, which filter network is connected in series with the load. The switched-mode power supply is lighter than the other power supplies which incorporate a transformer, and it is more efficient than the linear DC power supply. However, the switched-mode power supply is complex and therefore expensive. Switched-mode power supplies in the range of power ratings appropriate for energizing lamps, e.g., from around 100 watts to around 600 watts, cost on the order of $1.00 per watt, even in significant quantities. Accordingly, the cost of a switching power supply designed to drive a 400 watt lamp is approximately $400.00.
There is a fourth approach in the design of a power supply for controlling electrically energized lamps, but which is not suitable for driving low-voltage lamps from a high-voltage line. The fourth approach involves the use of a thyristor, such as a silicon-controlled rectifier (SCR) or triac circuit. The SCR is basically a gated diode. Like the conventional diode, the SCR blocks current in one direction (when "back-biased") but passes current in the other direction ("forward-biased"). Unlike the conventional diode, however, the SCR does not conduct in the forward-bias mode until it is "fired" by a signal applied to its control gate. If the SCR is fired at the beginning of a forwardbias input voltage cycle (i.e., the positive-going half of an AC sine wave), it will conduct the entire half cycle until the voltage drops to the zero-crossing line. At the zero-crossing, the SCR automatically turns "off," and it remains non-conductive until the next positive half-cycle and gate voltage command. The SCR can be turned on at any time during a positive-going half cycle to regulate the net time-averaged amount of power delivered to a load.
Accordingly, for each positive-going half cycle of the AC input voltage, the applied SCR gate voltage controls the phase angle, and therefore the time, at which the SCR will begin to conduct. The triac is similar to the SCR. However, the triac conducts in either direction when fired, and so it can be used to regulate power from both the positive-going and negative-going half cycles of the AC input voltage. Like the SCR, the triac stays in its conductive mode once fired, and automatically turns off at the zero-crossing point of the AC input voltage.
SCR and triac power supplies can be used, but they are not suitable to operate a low-voltage lamp from a high-voltage source. The reason for this is that neither power supply can be turned off on command, but must stay on until the next zero-crossing point of the AC voltage. However, SCR's and triacs may be used as efficient, low-cost dimmers to reduce the amount of light from a lamp which operates on the full 115 VAC line voltage.
The present invention provides a unique power supply having particular utility for powering a low-voltage incandescent lamp from a conventional 115 VAC line source without using either a transformer or a switched power supply technique. Accordingly, power supply of the present invention does not have the weight of the prior art transformer power supply circuits described above, or the cost, bulk and complexity of the prior art switched power supplies. The power supply of the present invention relies on the unique load characteristics presented by a low-voltage incandescent lamp, which will now be described.
High power, low-voltage incandescent lamps have heavy filaments, formed, for example, of tungsten wire having a relatively large cross section. This is because low-voltage operation requires lower filament resistance with correspondingly higher current. The corollary to heavy filaments is that their heat capacity, that is the product of the specific heat of tungsten multiplied by the mass of the filament, is high. High heat capacity creates a long thermal time constant of the order of seconds. A long thermal time constant means that the filament will not cool significantly during short (millisecond) modulated "off" times, or heat too rapidly during the "on" times. High heat capacity in a thermal system is analogous to a flywheel in a mechanical kinetic system, in that it resists rapid changes of its energy state. The heat capacity of the heavy tungsten filament acts as a thermal averager to smooth the fluctuations caused by temporal modulation.
The low-voltage tungsten lamp is effective in smoothing the effects of voltage fluctuation. This is also a corollary benefit of a long thermal time constant. An incandescent lamp is a near-classical black-body emitter. As a purely thermal device, the spectral content of its light is uniquely determined by its color temperature, and its gross light output is proportional to the emitting surface area of the filament. No matter how the tungsten lamp is driven, the characteristics of a particular lamp design are the same when the lamp reaches its specific design color temperature. For example, in the case of a 36-volt DC tungsten amp driven by an AC voltage, the peak voltage applied to the lamp oscillates between .+-.51 volts (36 volts multiplied by the square root of 2). When driven by 36 VAC (RMS), the light output of the lamp is the same as when it is driven by 36 volts DC. The only difference during AC operation is a slight 120 Hz modulation of the light output due to the filament cooling slightly below its nominal color temperature during the low-voltage portion of each cycle, and heating up slightly above its nominal color temperature during the maximum voltage peaks of the sine wave. In practice, this modulation is typically very small, of the order of 2% or 3%, and in any case it is not detectable by the human eye because the eye does not respond to flicker above about 20 Hz.
Of importance is that in order to operate a DC tungsten lamp by a sinusoidal AC voltage, the lamp must be capable of withstanding .sqroot.2 higher voltage (For example: 51 volts versus 36 volts) at the peak of the applied voltage without damage. Typically, such lamps are commonly designed to be operable on both AC and DC voltages.. The output of the lamps is the same under both AC and DC drives fundamentally because, as a thermal device, it responds to the mathematical root-mean-square of the applied voltage.
The fact that a 36-volt lamp will withstand a peak voltage of 51 volts when driven by a sinusoidal (60 Hz) AC voltage does not mean that the lamp will withstand an arbitrarily higher voltage, even if that voltage is applied over a short time. This is another unique property of the incandescent lamp as an electrical load. At the outset, it appears that there is no reason why a 36-volt incandescent lamp should not withstand a very high voltage applied for a very short time, for example, 200 volts for 500 microseconds, if the total input energy for that brief period is not sufficiently high instantaneously to heat the filament beyond its melting point. However, this premise is not valid because the incandescent lamp has a more subtle characteristic, namely, it functions as a hot cathode. As a hot cathode, the filament is surrounded by a cloud of free electrons. For a closely-wound filament, i.e., one with the individual turns of the tungsten helix closely adjacent to one another, there is a limiting voltage at which the turn-to-turn potential will break over with the current being carried not through the wire, but from turn-to-turn through the electron cloud. When this turn-to-turn potential is exceeded, the lamp impedance drops precipitously, the current drawn by the filament increases at a rapid rate, and some part of the filament melts from excessive current density. This non-linear response is the reason why the applied voltage must always remain below the critical "break-over" value.
The power supply of the present invention converts supplied low harmonic alternating current into controlled energy by "waveform slicing." The resulting voltage waveform is useful for illuminating the tungsten filament lamp referred to above, which has a rated working voltage range lower than that of the RMS (root-mean-square) voltage value of the supplied incoming waveform. The voltage across the lamp is switched off anytime the input voltage to the power supply exceeds a predetermined level, thus preventing voltage breakdown, or avalanching, between adjacent hot turns of the filament of the incandescent lamp.
The power supply of the invention permits conduction of during the leading (or rising edge) of each half cycle of the applied voltage waveform, and this conduction continues to a specific voltage level. Then, the power supply has the capability, as needed, to turn off the central section of the waveform and interrupt current flow. The power supply is then capable of turning the voltage back on when the voltage falls back down to a specific level, allowing current flow again, until the voltage waveform falls to zero volts.
When powered with conventional AC line voltage, the power supply of the invention allows current to flow twice during each half cycle of the sine wave, which results in twice the refresh frequency compared with a conventional triac type of controller and four times the refresh frequency as compared with the SCR type. This results in a lower peak filament voltage for equivalent RMS (root-mean-square) voltages from either prior art type of power supply. Furthermore, with the refresh frequency of the power supply of the invention being twice to four times that of the prior art type of controller, the power supply of the invention also produces lower peak-to-peak filament color temperature ripple. A slight asymmetry in the voltage waveform supplied to the lamp can reduce the peak-to-peak filament color temperature ripple even further.